1. Field of the Invention
The present invention relates to a valve timing control device for an internal combustion engine which controls valve timings of intake valves and exhaust valves, and more particularly, to a valve timing control device for an internal combustion engine which has a function of detecting an abnormality (malfunction) in a variable valve timing mechanism (VVT).
2. Description of the Related Art
Conventionally, there is well known a device for variably controlling a valve timing of at least one of an intake valve and an exhaust valve in accordance with an operational state. A valve timing control device of this type has a mechanism for variably maintaining and optimizing the valve timing in accordance with the operational state. For example, the timing for opening the intake valve is maintained on an advancement side for the purpose of realizing the function of exhaust gas recirculation (EGR) when the rotational speed of an engine is in a stable intermediate range, and maintained on a retardation side for the purpose of ensuring an output torque or stabilizing operation when the rotational speed of the engine is in a high rotation range or a low rotation range.
Driving performance and exhaust gas properties are influenced when an abnormality occurs in a variable valve timing mechanism (VVT), so a difference or the like between a target valve timing and an actual valve timing is observed to detect the abnormality in the variable valve timing mechanism. There is also proposed a device that detects an abnormality from a difference between a target valve timing and an actual valve timing but prohibits detection of an abnormality for a predetermined time after the start of an engine so as to prevent an erroneous determination from being made when the amount of hydraulic oil is insufficient at the start of the engine (e.g., see JP 2000-64862 A).
An initial value of a learning value, which is one of controlled variables of an actual valve timing, is set immediately after a turning-on of a power supply. It is proposed to set a value smaller than a designed median as the initial value such that the overlap amount of valves becomes relatively small, that is, in consideration of variations among parts, secular changes, or the like (e.g., see JP 08-338271 A).
Further, there is proposed a valve timing control device for an internal combustion engine which equalizes a learning value with a designed median and sets an initial value of an integral term immediately after the turning-on of the power supply to a negative value to make an actual valve timing converge swiftly at a target valve timing, and suppresses the overshoot of the actual valve timing to reliably prevent a degradation of driving performance and a deterioration of exhaust gas properties (e.g., see JP 2001-152886 A).
In each of the aforementioned conventional valve timing control devices, the learning value or the integral value of a controlled variable immediately after the turning-on of the power supply is set to a small value, so the convergence of the actual valve timing at the target valve timing is delayed until the learning of the controlled variable is completed. In making a determination on the occurrence of an abnormality in the variable valve timing mechanism, there is a problem in that it is erroneously determined in some cases that the variable valve timing mechanism is abnormal, despite normalness thereof, owing to a delay in convergence immediately after the turning-on of the power supply.